Thermal transfer sheet

Abstract: A thermal transfer sheet comprising a substrate film (1), optionally a release layer (3) provided on the substrate film and an ink layer provided on the substrate film or the release layer, the ink layer (3) comprising a colorant and a vinyl chloride/vinyl acetate copolymer resin having a Tg of 60 to 90°C and an average molecular weight of not less than 10,000.

Description: The present invention relates to thermal transfer sheets for thermal transfer printers used as a hard copy output device in personal computers, word processors and the like, and more particularly to thermal transfer sheets which can provide prints having excellent rubbing/scratch resistance and solvent resistance when printing is carried out on various plastics under high printing energy conditions. A hot-melt thermal transfer sheet formed by coating an ink comprising a mixture of wax with a pigment on one side (surface) of a substrate film by means of a coater to form a hot-melt ink layer has hitherto been widely used as a thermal transfer sheet at the time of printing of hard copies for personal computers, word processors and the like by the thermal transfer system. In the thermal transfer sheet having a thermal transfer ink layer composed mainly of wax, the thermal transfer sheet is imagewise heated by means of a thermal head from the back surface thereof to melt the thermal transfer ink in the thermal transfer ink layer. At that time, an image is formed on a material, on which an image is to be transferred, by taking advantage of the adhesive property of the ink layer developed by the heating. For this reason, the ink layer and the release layer each comprise a low-melting material. Due to the use of the low-melting material, prints formed by such a thermal transfer sheet have poor rubbing and scratch resistance. Further, the resistance of the prints to various general-purpose solvents is also poor. Therefore, it is difficult to use the above thermal transfer sheet in applications where the scratch resistance and the solvent resistance are required, particularly in printing on plastic labels, plastic cards, plastic bags and the like. On the other hand, printing under high printing energy conditions (high temperatures) has been proposed in order to enhance the fixability of the ink onto the surface of plastic materials. This method, however, can cause unfavorable phenomena, such as fusing of the release layer and further the substrate film onto prints due to the high temperature, tearing of the substrate film or occurrence of dropout, which is detrimental to the film formability of the surface of the print, thus resulting in a deterioration of the rubbing/scratch resistance and the solvent resistance of the prints. Japanese patent application JP-A 02 160 585 discloses an ink film which comprises a support and a porous resin layer in which a heat-fusible ink composition is impregnated, wherein the ink composition contains a CI-substituted alkylbenzamide, in order to improve the dispersion of a colorant, so that the concentration of the colorant is raised resulting in higher optical density prints. The porous resin layer may be of vinylchloride-vinylacetate copolymer having a Tg of preferably 100°C or higher. Japanese Patent Laid-Open No. 42891/1988 discloses a thermal printing medium comprising a substrate sheet, a transparent or semi-transparent protective layer provided on one surface of the substrate sheet and comprising a chlorinated polyolefin resin and an ink layer provided on the surface of the transparent or semi-transparent protective layer and comprising a mixture of a polymer of an acrylic or methacrylic ester with a colorant. This thermal printing medium is described to enable the formation of any image, such as bar codes and letters, on plastic articles, unattainable by the conventional thermal printing media. In this thermal printing medium, however, the transparent or semi-transparent protective layer is provided so that it is transferred together with the ink layer to a recording medium, on which an image is to be transferred, thereby protecting the surface of the transferred ink layer, and printing under high energy printing conditions is not taken into consideration. Accordingly, an object of the present invention is to solve the above-described problems of the prior art and to provide a thermal transfer sheet which can provide a good print even under high energy printing conditions (not less than 0.4 mJ dot), the print being excellent also in the rubbing/scratch resistance and solvent resistance. Another object of the present invention is to provide a thermal transfer sheet which can provide a good print on the surface of plastic materials (materials on which an image is to be printed), polyethylene terephthalate (PET), vinyl chloride and acrylic plastics, the print being excellent also in the rubbing/scratch resistance and solvent resistance. In order to attain the above objects, according to the present invention, there is provided a thermal transfer sheet comprising a substrate film and an ink layer provided on said substrate film, said ink layer comprising a colorant and a vinyl chloride/vinyl acetate copolymer resin having a Tg of 60 to 90°C and an average molecular weight of not less than 10,000. In the thermal transfer sheet of the present invention, the provision of an ink layer using as a binder a vinyl chloride/vinyl acetate copolymer resin having a Tg of 60 to 90°C and an average molecular weight of not less than 10,000 improves the compatibility of the ink with plastic materials (materials on which an image is to be printed), which contributes to an improvement in solvent resistance of the print. Fig. 1 is a cross-sectional view of an embodiment of the thermal transfer sheet according to the present invention. The thermal transfer sheet of the present invention will now be described in more detail with reference to the following preferred embodiments. Fig. 1 shows an embodiment of the thermal transfer sheet according to the present invention. The thermal transfer sheet comprises a substrate film 1 and a hot-melt ink layer 3 provided on the substrate film. The hot-melt ink layer 3 comprises a resin binder having a good compatibility with the substrate film 1 of a plastic material and an excellent solvent resistance. The thermal transfer sheet according to a preferred embodiment of the present invention comprises a substrate film 1, a back surface layer provided on the back surface of the substrate film 1, and a protective layer provided between the substrate film 1 and a hot-melt ink layer 3. The back surface layer is a heat-resistant protective layer which serves to impart sufficient lubricity to a thermal head and, at the same time, to prevent deposition of contaminants on the thermal head. The protective layer serves to impart resistance to plasticizers, rubbing/scratch resistance and solvent resistance after printing to the print. Substrate film In the present invention, the substrate film 1 used in the present invention is not particularly limited and may be the same as the substrate film used in the conventional thermal transfer sheets. Specific preferred examples of the material for constituting the substrate film 1 include films of plastics, such as polyesters, polypropylene, cellophane, polycarbonates, cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, nylons, polyimides, polyvinylidene chloride, polyvinyl alcohol, fluororesins, chlorinated rubber and ionomers, various types of paper, such as capacitor paper and paraffin paper, and nonwoven fabrics. Further, composite materials comprising a combination of the above materials may also be used. The thickness of the substrata film 1 may be properly selected depending upon materials used so that the strength and the thermal conductivity of the substrate film are proper. For example, it is preferably in the range of from about 2 to 25 µm. If necessary, a back surface layer comprising a heat-resistant resin and a thermal release agent or a lubricant may be provided on the back surface of the substrate film 1 for the purpose of rendering the thermal sheet smoothly slidable and, at the same time, preventing sticking. Hot-melt ink layer The hot-melt ink layer (3) is provided on the substrate film 1 and the thickness thereof is preferably in the range of from about 0.5 to 5.0 µm. The hot-melt ink layer (3) comprises a resin component as a binder and a colorant and, if necessary, various additives. Examples of the resin component as the binder include ethylene/vinyl acetate copolymer resin, ethylene/ethyl acrylate copolymer resin, polyamide resin, polyester resin, epoxy resin, polyurethane resin, acrylic resin, vinyl chloride resin, cellulosic resin, polyvinyl alcohol resin, petroleum resin, phenolic resin, styrene resin, and elastomers, such as natural rubber, styrene/butadiene rubber, isoprene rubber and chloroprene rubber. Among them, resins and elastomers having a softening point in the range of from 50 to 150°C and an average molecular weight in the range of from 5,000 to 50,000 are preferred. The resin component as the binder preferably has a Tg of 60 to 90°C and an average molecular weight of not less than 10,000 from the viewpoint of preventing occurrence of blocking when the thermal transfer sheet is taken up into a roll. Particularly preferred is a vinyl chloride/vinyl acetate copolymer resin having a Tg of 60 to 90°C and an average molecular weight of not less than 10,000. Moreover, waxes, amides, esters or salts of high fatty acids, fluororesins, powders of inorganic substances and the like may be added as an anti-blocking agent. The colorant may be properly selected from known organic or inorganic pigments or dyes. For examples, it preferably has a sufficient color density and neither discolors nor fades upon exposure to light, heat and the like. Further, it may be a material which develops a color upon heating or upon contact with a component coated on the surface of a material to which an image is to be transferred. Moreover, the color of the colorant is not limited to cyan, magenta, yellow and black, and colorants of various other colors may be used. In the ink layer, the weight ratio of the resin component to the colorant is preferably in the range of from 30 : 70 to 95 : 5, still preferably in the range of from 40 : 60 to 90 : 10. Protective layer In the thermal transfer sheet of the present invention, if necessary, a protective layer composed mainly of PMMA (a polymethyl methacrylate resin) may be provided between the substrate film 1 and the hot-melt ink layer 3. The protective layer serves to impart resistance to plasticizers, rubbing/scratch resistance and solvent resistance after printing to the resultant print. Polyethylene wax may be added in an amount of 0 to 20% by weight, preferably about 10% by weight, to the protective layer 5 for the purpose of enhancing the rubbing/scratch resistance. Further, in order to enhance the adhesion of the protective layer, it is also possible to add to the protective layer ethylene/vinyl acetate copolymer resin, polyesters, acrylic resin and other resins in an amount of 0 to 20% by weight, preferably about 10% by weight. The thermal transfer sheet of the present invention may be prepared by successively forming the above-described intended layer(s) on a substrate according to any conventional method commonly used in the art. For example, it may be formed as follows. Components for constituting an intended layer, together with optional additives, are added to and dissolved or dispersed in a suitable solvent, if necessary, using a dispersing device, such as an attritor, a ball mill or a sand mill, to prepare a coating solution in the form of a solution or a dispersion. The coating solution is coated by means of a coater, such as a gravure coater or a roll coater, and the resultant coating is then dried. If necessary, the above procedure is repeated for successively forming the other intended layers. Thus, the thermal transfer sheet of the present invention can be provided. The present invention will now be described in more detail with reference to the following examples, though it is not limited to these examples only. Example 1 A 4.5 µm-thick polyethylene terephthalate film (Lumirror manufactured by Toray Industries, Inc.) was provided for use as a substrate film, and an ink having the following composition for a back surface layer was coated on one surface of the substrate film and dried to form a back surface layer. Then, the following components of an ink composition for an ink layer were dispersed in one another by means of an attritor as a dispersing device to prepare a coating solution for an ink layer. The coating solution was coated on the surface of the substrate film remote from the back surface layer at a coverage of 0.8 g/m 2 by means of a gravure coater as a coating device to form a hot-melt ink layer, thereby preparing the thermal transfer sheet of the present invention. Ink for ink layer Vinyl chloride/vinyl acetate copolymer resin Tg: 68°C Average molecular weight: 15,000 15 parts by weight Carbon black 10 parts by weight Toluene 75 parts by weight Comparative Example 1 A thermal transfer sheet sample was prepared in the following manner, except that a vinyl chloride/vinyl acetate copolymer resin having a Tg of 55°C and an average molecular weight of 27,000 was used instead of the vinyl chloride/vinyl acetate copolymer resin: A 6 µm-thick back coated film K200S6E for thermal transfer (a film with a back surface layer provided thereon, manufactured by Diafoil Hoechst Co., Ltd.) was provided for use as a substrate film. Then, a release layer, a protective layer and an ink layer respectively having the following compositions were formed in that order on the surface of the substrate film remote from the back surface layer by coating in the following manner, thereby preparing a thermal transfer sheet. The following following components of an ink composition for a release layer were dispersed in each other by means of an attritor as a dispersing device to prepare a coating solution for a release layer. The coating solution was coated on the other surface of the. substrate film remote from the back surface layer at a coverage of 0.3 g/m 2 by means of a gravure coater as a coating device to form a release layer. Ink for release layer: coverage 0.4 g/m 2 Chlorinated polypropylene Chlorine content: 64% by weight Average molecular weight: 75,000 Melting point: 180°C 30 parts by weight Toluene 35 parts by weight MEK 35 parts by weight Ink for protective layer: coverage 1.0 g/m 2 Polymethyl methacrylate (PMMA) Tg: 105°C Average molecular weight: 40,000 30 parts by weight Toluene 35 parts by weight MEK 35 parts by weight Ink for ink layer: coverage 0.9 g/m 2 Vinyl chloride/vinyl acetate copolymer Tg: 68°C Average molecular weight: 15,000 Vinyl chloride/vinyl acetate: 82/18 12.5 parts by weight Toluene 40 parts by weight MEK 35 parts by weight Carbon black 12.5 parts by weight Comparative Example 2 A vinyl chloride/vinyl acetate copolymer resin having a Tg of 90°C and an average molecular weight of 10,000 was used. However, the dissolution thereof was so difficult that an ink could not be prepared. Comparative Example 3 A thermal transfer sheet sample was prepared in the same manner as in comparative Example 1, except that a vinyl chloride/vinyl acetate copolymer resin having a Tg of 65°C and an average molecular weight of 8,000 was used instead of the vinyl chloride/vinyl acetate copolymer resin used in comparative Example 1. Comparative Example 4 A thermal transfer sheet sample was prepared in the same manner as in comparative Example 1, except that an acrylic resin (Tg: 60°C, average molecular weight: 30,000) was used instead of the vinyl chloride/vinyl acetate copolymer resin used in comparative Example 1. The thermal transfer sheets thus obtained were used to print a bar code pattern on three types of plastic films, that is, polyvinyl chloride, polyethylene terephthalate (PET) and acrylic films, by means of a bar code printer BC8MK manufactured by Auto Nics Co., Ltd. (printing energy: 0.352 mJ/dot). The printed bar codes were subjected to the following tests to evaluate the quality of the prints. The results are given in Table 1. Printability When the print was scanned with AUTOSCAN manufactured by RJS, ○: successful reading X: failure of reading Rubbing/scratch resistance Apparatus: HEIDON-14 manufactured by HEIDON Load: 300 g (rubbing/scratching with a stainless boll under this load) Rate of travel: 6,000 mm/min Number of times of rubbing/scratching: 40 Chemical Resistance The sample was immersed in denatured ethanol for 5 min and then subjected to a test under the same conditions as those described above in connection with the rubbing/scratch resistance test. After the above rubbing/scratch resistance and chemical resistance tests, the bar codes were again read with AUTOSCAN to measure the reflectance. When the difference in reflectance between before the test and after the test was 5 or less, the property was evaluated as ○, while when the difference exceeded 5, the property was evaluated as X. Further, the thermal transfer sheets prepared above were evaluated for storage stability under the following storing conditions. The results are also given in Table 2. Criteria of evaluation for storage stability: ○: No offset observed X: Offset observed Evaluation conditions for storage stability: The thermal transfer sheet was subjected to ribboning, stored in this state at a temperature of 55°C and a humidity of 85% for 24 hr and then evaluated.

Claims: A thermal transfer sheet comprising a substrate film (1) and an ink layer (3), provided on said substrate film (1), said ink layer (3) comprising a colorant and a vinyl chloride/vinyl acetate copolymer resin having a Tg of 60 to 90°C and an average molecular weight of not less than 10,000. A thermal transfer sheet according to claim 1, wherein a back surface layer is provided on the back surface of the substrate film (1). A thermal transfer sheet according to claims 1 or 2 wherein a protective layer is provided between the substrate film (1) and the ink layer (3). A thermal transfer sheet according to any one of claims 1 to 3, which is used under a high printing energy condition (not less than 0.4 mJ/dot).

Inventors: HIROSE KEIJI, OGAWA KEIICHI, HIROSE, KEIJI, OGAWA, KEIICHI, HIROSE, KEIJI, C/O DAI NIPPON PRINTING CO., LTD., OGAWA, KEIICHI, C/O DAI NIPPON PRINTING CO., LTD.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON PRINTING CO., LTD.
Application year: 1994
Main classification: B41M5
IPC codes: B41M5